Blood oxygen saturation is the relative amount of oxygenated hemoglobin in all of the hemoglobin present in the blood stream. This hemoglobin is packaged in biconcave discs of approximately 10 micrometers diameter which commonly occur with a density of approximately five million red blood cells per cubic millimeter. The red blood cells both scatter and transmit the incident radiant energy. The differential absorption by oxygenated and non-oxygenated hemoglobin of the radiant energy transmitted through the red blood cells furnishes the basis for the oxygen saturation measurement. In conventional catheter oximetry, the radiation wavebands being used to measure the blood at a measurement site in vivo are conducted from the oximeter device to the position of interest within the flowing blood stream by means of an optical catheter including light-transmitting and light-receiving fiberoptic light guides. The receiving fiberoptic light guide for conducting light from the blood stream back to a photodetector in the oximeter device commonly has its inlet aperture coplanar with the outlet aperture of the transmitting fiberoptic light guide. Thus, only back-scattered light is available for measurement, and this represents only a very small fraction of the total light transmitted to the measurement site. The light scatterers present about the measurement site thus act as sources of light for the receiving optical light guide. Consequently, the intensity of the light scattered back to the receiving optical light guide is influenced by variations in the number of scatterers, their location, size, shape and orientation as well as by the differential absorption by oxyhemoglobin and hemoglobin.
The blood under test flows within a vessel of interest in a pulsatile manner, and the catheter tip thus moves in an uncontrolled manner with respect to the blood vessel walls. Whenever a blood vessel wall appears in the near field of the catheter tip, this has the effect of introducing a very large array of tightly packed backscatterers into the measurement system. This introduces a significant change in the distribution and number of scatterers, which has a substantial and wavelength-dependent effect upon intensities of light received by the receiving fiber as a function of transmission through hemoglobin and oxyhemoglobin (which have wavelength-dependent radiation absorption characteristics).
Certain known catheter-type oximeter devices respond to the intensities of such back-scattered radiation at only two different wavelengths. Oximeter devices of this type are disclosed in the literature (see, for example, U.S. Pat. No. 3,847,483 issued to R. F. Shaw, et al, on Nov. 12, 1974). In these known devices, the radiation intensities measured at two wavelengths provide an indication of oxygen saturation according to the relationship: ##EQU1## where I.sub.1 and I.sub.2 are the light intensities measured at wavelengths .lambda..sub.1 and .lambda..sub.2, respectively.
It should be noted that if both the numerator and denominator of equation 1 are divided by one of the light intensity measurements, i.e., I.sub.1, the resultant expression is ##EQU2## Because the OS measurement thus made according to the prior art remains a function not only of the ratio of light intensities but of individual light intensities as well, variations in such phenomena as blood flow velocity, hematocrit, pH, pCO.sub.2, and the like (which are multiplicative and wavelength dependent), can introduce errors into the oxygen saturation measurement thus obtained.
The apparatus of the aforecited patent exhibits greater immunity to such sources of error as variations in blood flow velocities, hematocrits, and hemoglobin concentrations than apparatus previously known. However, even greater immunity is desirable to such sources of error in applications requiring high-accuracy in vivo measurements of oxygen saturation. In particular, less sensitivity to proximity of the catheter tip to blood vessel walls is preferable. In addition, a detectable influence upon measurement accuracy due to variations in hematocrit, flow velocity, pH, pCO.sub.2, osmolarity, and variations in the transmissivity of the optical fibers is also present, and, to some extent, error also can result from a linear characterization of nonlinear phenomena in the prior art.